Comparative cranial morphology in living and extinct platypuses: Feeding behavior, electroreception, and loss of teeth.

Abstract

The modern platypus, <i>Ornithorhynchus anatinus</i>, has an eye structure similar to aquatic mammals; however, platypuses also have a "sixth sense" associated with the bill electro- and mechanoreception that they use without opening their eyes underwater. We hypothesize that <i>Ornithorhynchus</i> and the Miocene taxon <i>Obdurodon</i> have different sensory capacities, which may have resulted from differences in foraging behavior. To estimate differences in foraging, sensory systems, and anatomical divergence between these monotremes, we compared their skull morphologies. Results indicate that the bill of <i>Obdurodon</i> is more dorsally deflected than that of <i>Ornithorhynchus</i>, suggesting a pelagic foraging behavior in <i>Obdurodon</i> compared to the bottom-feeding behavior in <i>Ornithorhynchus</i>. The infraorbital foramen of <i>Obdurodon</i>, through which the maxillary nerve passes sensory data from the bill to the brain, is relatively less developed than that of <i>Ornithorhynchus</i>. Whereas bill-focused sensory perception was likely shared among Mesozoic monotremes, the highly developed electrosensory system of <i>Ornithorhynchus</i> may represent an adaptation to foraging in cloudy water. Computed tomography imagery indicates that the enlarged infraorbital canal of <i>Ornithorhynchus</i> restricts the space available for maxillary tooth roots. Hence, loss of functional teeth in <i>Ornithorhynchus</i> may possibly have resulted from a shift in foraging behavior and coordinate elaboration of the electroreceptive sensory system. Well-developed electroreceptivity in monotremes is known at least as far back as the early Cretaceous; however, there are differences in the extent of elaboration of the feature among members of the ornithorhynchid lineage.